Most devices include an element converting energy. Particularly, in a case of a sensor, an electrical output is mostly required such that at least one energy conversion is processed within the device. For example, in a case of a capacitive type of microphone that is widely used, displacement of a thin film that is deformed by a pressure wave of a sound changes a capacitance with an electrode positioned near the thin film, and the change of capacitance changes an electrical state that was formed in a balance state by a bias voltage that is initially applied, thereby generating a voltage change or a charge of change amount between the thin film and the electrode. If they are connected to an electric circuit to sense the change of the capacitance, the displacement of the thin film may be estimated therefrom, so this method is connected to the estimation of the sound pressure. Accordingly, correctly converting the change of capacitance into the electrical output is important. To change the electrical state change of the electric element into the electric output, a current is needed such that the correct conversion of the capacitance change into the electrical output signal has a limitation. Particularly, a case using reversible energy conversion may not escape this characteristic. The reversible energy conversion means that an energy conversion direction is a pair of directions, and this includes conversion using a condenser added with the above-proposed bias voltage as well as conversion using a piezoelectric phenomenon.
The energy conversion method that is widely used in the current sensor may be largely classified into a piezoresistive type, a piezoelectric type, an optical type, and a capacitive type. In the piezoelectric type and the capacitive type, the electrical signal corresponding to the final output next to the reversible energy conversion is not free in the dynamics of the sensor. In the piezoresistive type and the optical type, non-reversible energy conversion is easy such that the dynamic of the sensor does not largely affect the accuracy of the signal. Accordingly, in the case of the piezoresistive type of sensor, since the electrical impedance of the mechanical-electrical conversion material is low, there is a merit that an additional interface circuit for the signal measurement of the piezoresistive type of sensor is not needed and the change of the sensitivity according to a frequency is small. Also, a MEMS process technique has been developed for the production to be easy according to the development of a new piezoresistive material, and various researches on the application of the very small pressure sensor and the microphone have been executed. However, due to the low mechanical-electrical conversion efficiency of the piezoresistive material, there is a drawback that the entire sensitivity is relatively low compared with other methods, and a change of the sensitivity of the sensor is largely generated according to temperature.
In the case of the piezoelectric type of sensor, since the output signal having a relatively large size may be obtained without the addition of the bias voltage, it has been largely applied to a sound sensor such as the microphone or a hydrophone until now, however it is the reversible conversion method such that it may be not free from the unique characteristic. Accordingly, since the size of the piezoelectric member used is minutely small if extreme down-sizing is realized, the charge amount that may be generated by the pressure is extremely small such that the conventional circuital measuring method may not satisfy the required sensitivity, the frequency band, and a noise for the signal. Particularly, an increase of electric impedance according to the down-sizing causes deterioration of the low frequency performance.
The capacitive type of sensor is widely applied to a very small pressure sensor, sound sensor, and acceleration-based sensor, however it is the reversible energy conversion like the piezoelectric sensor such that the capacitance of the sensor is extremely small according to the extreme down-sizing, and the sensitivity deterioration effect due to parasitic capacitance appears. To prevent this, the capacitive type of very small sensor integrates the advanced and high performance interface circuit with a single chip shape and is realized with a hybrid packaging method minimizing the parasitic capacitance, according to the high performance and CMOS IC integration of the low noise, and the packaging technique is perched as an important element technology. However, although the sensor and the interface circuit are integrated and the sensitivity is preserved, the capacitive type of sensor may not avoid the deterioration of the sensitivity of the low frequency region and the noise characteristic according to the extreme down-sizing. It is difficult to solve the problem of the low frequency characteristic and the sensitivity due the technical and electrical conversion method of the sensor through the high performance and the optimization of the interface circuit.
The optical type of sensor uses a method of directly measuring mechanical deformation of the sensor sensing portion by an optical method such as Mach-Zehnder interferometry and Fabry-Perot interferometry. The piezoresistive type and this type use non-reversible energy conversion methods such that the limitation of the output signal is not determined by the dynamic of the sensor. However, the optical type of sensor has drawbacks such as the change of sensitivity according to the sensor temperature and the difficult aligning and calibration of optics in spite of the several merits such as the sensitivity.
As described above, the capacitive type and the piezoelectric type adopted in the conventional sensor have the limitation such as the deterioration of the sensitivity and the low frequency performance according to the extreme down-sizing, and the optical type and the piezoresistive type do not have the deterioration of the frequency performance according to the down-sizing, but are different to be itself down-size or have the problem a noise ratio for a signal does not reach a required degree.